1. Field of the Invention
This invention relates to a semiconductor laser element and a driving method therefor, and particularly, a semiconductor laser element which emits laser light at different wavelengths by controlling the amount of current flowing through the element, and its driving method.
2. Related Background Art
In recent years, demands for semiconductor laser elements in the field of optical communication or optical information processing have rapidly increased and with that, requests to functions of the elements have become diversified. Semiconductor laser elements of which oscillation wavelength is variable are not an exception. For example, in a case where recording medium such as an optical card or an optical disk, etc. is subjected to laser light to record and reproduce information to and from the recording medium, writing-in due to reproduced light is usually prevented by lowering the power of the output of the reproduced light with respect to recording light. At that time, if the semiconductor laser element of which variable oscillation output wavelength is used and the wavelength of the reproduced light is set to a value which is low in medium sensitivity, then it is possible to avoid the above-mentioned writing-in without substantially lowering the output of the reproduced light, and thus, to provide reproduction of information which is higher in S/N ratio.
For the above-mentioned, requests, as one a first example of prior art, a technique has been proposed in which light emitting layers emitting lights of different wavelengths, respectively are formed on respective separate light directing paths on the same substrate, so that a light emitting layer of a selected wavelength can be caused to laser oscillate by a current injected independently, as shown in Appi. Phys. Lett. Vol. 36, p.442 (1980), for example. This technique provides essentially independent laser elements formed on the same substrate.
On the other hand, as a second example of prior art, what is called a distribution Bragg reflection (DBR) semiconductor laser utilizing a grating as a reflector constituting a resonator has been proposed. This is an element such that an electrode is provided on the grating portion, so that a carrier can be injected, and the oscillation wavelength is varied with the varied refractive index of the grating portion by controlling the amount of the injected current thereinto. In this case, the structure of the light emitting layer, etc. is the same as that in the usual semiconductor laser.
Also, as a third example of prior art, an element has been proposed such that a single quantum well is used as the light emitting layer, light emission from a high-order quantum level, is enabled with the increase of resonator loss, and laser oscillation at different wavelengths is obtained by the light emission from a first quantum level and a second quantum level.
FIG. 1A shows energy bands in such single quantum well and FIG. 1B shows its gain spectrums. In the conventional laser having the usual resonator loss, oscillation threshold gain is Gth , the gain spectrum presents the peak at wavelength .lambda..sub.1 for the first quantum level, and the light at this wavelength .lambda..sub.1 oscillates. In this third prior art example, when the oscillation threshold gain is made to Gth' with the increase of the resonator loss of this laser, the oscillation of the light at wavelength .lambda..sub.2 corresponding to the second quantum level is enabled.
Further, as a fourth example of prior art, U.S. Pat. No. 4,817,110 discloses an element such that two different quantum wells each having an oscillation wavelength of which the difference from the other is small are used as the light emitting layers, and the laser oscillation at the different wavelengths is obtained by the light emission from the respective quantum wells.
However, the above-mentioned prior arts examples have the following problems.
First of all, in the first prior art example, as the wavelength is changed, the light emitting position from the laser varies. For this reason, for example, when an external optical system is assembled so that it can focus light at one wavelength to a point, the focus position would shift by a far distance in comparison with a small shift due to wavelength dispersion if that wavelength is changed. Also, since it needs to drive independently a plurality of individual laser elements formed on the same substrate, producing processes become complicated and difficult, and the size of the element becomes large.
Next, in the second prior art example, the variable wavelength range thereof is narrow. For example, it is only about several nm in a laser using Al.sub.x Ga.sub.1-x As. This is because in the usual semiconductor laser the variable wavelength width depends on the change of Bragg wavelength due to the control of the amount of injected current, and thus, the former is limited to the latter.
Also, in the third prior art example, since it is based on the increase of the resonator loss, it has disadvantages of low efficiency of the laser, large value of the oscillation threshold current and small output, etc. Accordingly, with this element, it is impossible to obtain a two-wavelength laser which enables continuous oscillation at room temperature.
Lastly, in the fourth prior art example, there is not a substantial difference between the two oscillation wavelengths. That is to say, the variable wavelength range is narrow.